The coal gasification process involves turning coal or other carbon-containing solids into synthesis gas. While both dry coal and a water slurry can be used in the gasification process, dry coal pumping is more thermally efficient than current water slurry technology. A wide variety of equipment has been used for pumping particulate material. Such transport equipment includes conveyor belts, rotary valves, lock hoppers, screw-type feeders, and the like.
One of the devices currently being used to pump dry coal to a high pressure is the cycling lock hopper. While the thermal cold gas efficiency of cycling lock hopper fed gasifiers is higher than other currently available water slurry technology in the gasification field, the mechanical efficiency of the cycling lock hopper is relatively low. The capital costs and operating costs of cycling lock hoppers are also high due to the high pressure tanks, valves, and gas compressors required in the cycling lock hopper process.
The use of dry coal extrusion pumps has become more common in dry coal gasification. However, some of the problems associated with currently available dry coal extrusion pumps are internal shear failure zones and flow stagnation problems. The presence of failure zones can lead to a decreased mechanical efficiency in the pump, as they result in a loss of the ability to transmit forces from the mechanical drive into transport of the particulate material.
For example, in dry coal extrusion type pumps such as rotary disk type pumps, particulate material enters a transport duct between two drive disk walls and is driven by movement of the drive walls from an inlet toward an outlet. The movement of the drive walls compacts the particles such that the particles transmit stresses across contacts with each other and the particulate material engages the drive walls, resulting in a drive force being transferred from the drive walls to the particulate material. As the particulate material enters the transport duct, it should be sufficiently compacted or compressed prior to or upon entry into the pumping apparatus to cause the particles to transmit stresses across their contacts, resulting in the formation of, what is referred to herein as, a transient solid or bridge composed of compacted particulate material that allows the solids pump to develop head or pressure in the particulate material and effectively convey the particulate material through the solids pump to a region of higher pressure. Successive bridges should occur cumulatively within the transport duct as further particulate material enters the inlet.
Fine particulate and powdery materials, such as dry pulverized coal, are difficult to effectively convey through the pumping system. Fine particulate and powdery materials tend to be aerated or be well mixed with a gas when transported loosely or when loosely dropped through the inlet. The aerated fine particulate and powdery material may not be compacted enough to form a stress transmitting bridge of contacting particles between the rotary disks of the pumping device. As a result, the frictional force acting on the material by the rotary disks is not enough to transfer drive force to the material. Consequently, the fine particulate and powdery material may slip between the rotary disks and may not be effectively conveyed through the pumping device. If too much external force is applied to attempt to compress or deaerate the powdery material, the material tends to overly consolidate, clogging the inlet or the transport channel.
Thus, there is a need in the industry for an effective particulate transporting system for efficiently transporting fine particulate materials, particularly pulverized coal, using dry solids pumps that require particle bridging that allows the pumps to develop head or pressure in the particulate material.